1. Field of the Invention
The present invention relates to a method for making an optical fiber, and more particularly to a method for monitoring a spin imparted on an optical fiber to reduce Polarization Mode Dispersion(PMD) and ensuring bidirectional symmetry of the spin.
2. Description of the Related Art
A single mode optical fiber with a circular symmetric structure theoretically has two orthogonal polarization modes which are compensated each other. Generally, an electric field of the light propagating through an optical fiber can be considered as linear overlap of such two peculiar polarization modes. In the single mode optical fiber in fact, compensation of the two polarization modes is generated due to defective factors such as symmetric lateral stress or eccentricity of a circular core.
These two modes are propagated at different phase rates, hence two modes have different propagation constants (β1 and β2). This difference of propagation constants is called double refraction (Δβ), and the increase of double refraction means the increase of rate difference between two polarization modes.
Differential time delay between two polarization modes is called Polarization Mode Dispersion (hereinafter, referred to as “PMD”). The presence of PMD is one of factors causing difficulty in high-speed transmission or analog data transmission.
To improve PMD characteristics of the optical fiber, there are significantly handled an optical fiber preform making technique and an optical fiber drawing technique.
The optical fiber preform making technique is directed to lowering PMD of the optical fiber drawn from the preform by increasing the non-circularity of preform.
In addition, the optical fiber drawing technique is directed to lowering PMD by making the optical fiber be twisted with a pitch far less than its beat length so that the polarization modes can be gradually compensated due to relative delay between the modes.
U.S. Pat. Nos. 5,298,047 and 5,418,881 by Hart et al. disclose a method for applying a torque to an optical fiber so that a spin imparted on the optical fiber has non-constant spatial frequency (spins/m) by oscillating a guide roller contacted with the coated optical fiber at a certain angle with respective to a drawing axis or linearly reciprocating the guide roller to a direction perpendicular to the drawing axis. Particularly, Hart el al. report that a spin function applying a torque to the optical fiber is desirably a sine function which imparts alternating spins clockwise and counterclockwise to the optical fiber.
At this point, U.S. Pat. Nos. 5,943,466 and 6,240,748 by Henderson et al. report that the spin function generating a torque to an optical fiber strand is substantially not a sine function, and a time-varying complex function having at least two peak values such as frequency-modulated sine function or amplitude-modulated sine function is more effective to achieve reduction of PMD.
In addition, U.S. Pat. Nos. 5,897,680 and 6,148,131 by Geertman disclose a method for arranging a pair of rotating rollers to be faced each other on the basis of a drawing axis, contacting an optical fiber to the rollers respectively so that the rollers press optical fiber strands, and then rotating the rotating rollers on the center of the optical fiber in order to apply alternating torques clockwise and counterclockwise to the optical fiber.
The conventional spin imparting methods described above are reported to be effective for reduction of PMD in an optical fiber drawing process.
However, even in the above conventional techniques, it is also impossible to measure whether the spin imparted on the optical fiber in the actual process satisfies predetermined spin rates (spins/m) and spin period or not.
But, reliability of the spin imparting function is indirectly determined by measuring PMD of the optical fiber wound around a spool after the drawing process.
The reason that determining whether the spin is reliably imparted during an actual process in the conventional art is impossible is closely related to a spin generating principle of a spin imparting device. In other words, the conventional spin imparting device installs rollers on a path of the optical fiber drawn at a high speed, and then moves the rollers independently of the movement of optical fiber to generate a spin. Thus, the factors affecting on spin generation tend to be easily varied according to process conditions. Therefore, in the conventional art, it is substantially impossible to monitor and measure whether the spin function set at an initial stage of the process is properly operated through an actual process.
Thus, there is still required a method for monitoring a pattern of a spin imparted on the optical fiber during the process execution, determining whether spin period and spin rate based on the spin function are suitably practiced, and controlling the spin period and the spin rate.